24 November 2014
BUILD IT YOURSELF
By Gordon Hoffman
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When I pick a zener diode from my parts
cabinet, I always want to check its
breakdown voltage before I use it in a
project — just to be sure there wasn't a
mix-up in my cabinet drawer. This
procedure needs to be repeated when
putting unused diodes back into my
cabinet, or when sorting among a bunch
of diodes I picked up at a surplus outlet.
It's time consuming to set up a power
supply and voltmeter, select a series
resistor to limit current, and measure the
voltage for each diode. Alternatives like
reading the part number and looking it
up, or setting up a curve tracer aren't any
faster. Many errors, burned out diodes,
and damaged projects have convinced
me there must be a better way. While
experimenting with self-oscillating
switching power supplies for another
application, it dawned on me that this
technology was the answer to my
problem.
ZENER DIODE
TESTER
The tester described in this article is a simple two-transistor circuit operating from a 9V battery which tests zener
diodes with breakdown voltages up to
52 volts. Nothing more than a multimeter is
necessary to build, test, and use the circuit. Its
transformer-coupled design adjusts the output
voltage automatically to the voltage of the zener
diode under test, while adjusting diode current to
preserve a relatively constant diode power
throughout the measurement range.
